An optical pickup applies focused laser light onto a recording track, formed on an information recording surface of an optical recording medium, to form a spot of light on the recording track, so that the optical pickup can detect a change in the light reflected from the information recording surface to thereby read the information recorded on the surface. Therefore, it is necessary that the information reading laser light is always focused on the recording track even when the recording surface of the recording medium is displaced out of position due to a warp of the recording medium, etc. To achieve this, the optical pickup apparatus includes a focus servo for focusing the laser light by moving precisely an objective lens in a direction perpendicular to the information recording surface. Also, since it is necessary to have the laser beam accurately track the recording track even when the recording track is eccentric, the optical pickup apparatus also includes a tracking servo for precisely moving the laser spot in a direction perpendicular to the recording track, i.e., the radial direction.
FIG. 5 shows a conventional drive device for driving an objective lens to perform the focusing servo and tracking servo operations. This conventional drive device is disclosed in U.S. Pat. No. 4,861,138, issued to Suzuki on Aug. 29, 1989.
As shown in FIG. 5, a tracking coil 3 is fixedly secured to one end of a holder member 2 for holding an objective lens 1. The holder member 2 is supported by a support mechanism (not shown) in such a manner that the holder member 2 is movable in a direction (indicated by an arrow T) perpendicular to the direction (indicated by an arrow F) of the optical axis of the objective lens 1; that is, movable in a direction perpendicular to a recording track of a recording medium (not shown).
A magnet 4 in the form of a rectangular plate is disposed in opposed relation to the tracking coil 3 and is spaced a predetermined distance from the tracking coil 3. The magnet 4 is affixed to a fixed support member 5. The magnet 4 is so magnetized that it has two pole surfaces 5a and 5b of opposite polarity (north and south poles) juxtaposed in the direction of the arrow T. The tracking coil overlaps both of these two poles astride the boundary therebetween.
A magnetic flux designated by reference numeral 6 is produced in the drive device of the above construction, and when electric current is supplied to the tracking coil 3, an electromagnetic force is exerted in the direction of the arrow T on those two portions of the tracking coil 3 facing the north pole and south pole surfaces, respectively. The objective lens 1 is driven, together with the holder member 2, by this electromagnetic force to thereby effect the tracking servo.
In the above drive device in which a magnetic flux is produced between the two pole surfaces of opposite polarity disposed adjacent each other in a plane, the magnetic circuit is very thin, and therefore the overall construction of the device can be compact. However, the drive device requires a large and relatively expensive magnet, and therefore the overall cost is high. The magnet of such a construction is produced by magnetizing a flat plate of magnetic material using a magnetizer having two magnetizing yokes of opposite polarity disposed in juxtaposed relation to each other. With this method, however, the two pole surfaces can not be perfectly magnetized in the vicinity of the boundary therebetween, and to compensate for this, the magnet must be increased in size. This increases the size of the tracking coil, and offers a problem to be solved in achieving a compact overall construction of the device.